Subsurface valves are generally of the hydraulically operated spring loaded rod/piston type for use in the downhole environments of wellbores to regulate the flow of production fluids through the well. The valves provide barriers to restrain the uncontrolled flow of the fluid in the tubing string. Such valves generally provide regulation of fluid flow in the uphole direction as a result of pressure release from a production zone, but may not be adequately operable at extreme depths as a result of an excessive hydrostatic head in the tubing string.
A conventional valve incorporates a flapper mechanism biased to a normally closed position by a spring. Such a flapper mechanism is opened by the application of hydraulic control pressure to a piston that actuates the valve and positions it in an open position. If the hydraulic control pressure is lost, then the valve closes.
Control of such valves is, however, limited by the hydrostatic force applied to the piston. The hydrostatic force applied by the column of fluid in the control line varies with the depth at which the valve is positioned while the counteracting spring force biasing the valve closed is constant. The operability of the valve is, therefore, a function of its location in the well. If the valve is positioned at a depth such that the hydrostatic pressure generated by the column of fluid in the control line or tube is greater than the biasing force exerted by the spring mechanism, the valve will not close in response to a decrease in control pressure.